Design of Forced Air-cooling Structure for Elevated Temperature PEMFC

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1 Page EVS25 Shenzhen, China, Nov 5-9, 2010 Design of Forced Air-cooling Structure for Elevated Temperature PEMFC Xiaoliang Zheng 1,2, Daijun Yang 1, Kan Tao 2, Hao Zhang 1,2 and Jianxin Ma 1,2,* 1 School of Automotive Studies, Tongji University, 4800 Cao an Road, Shanghai, , P R China jxma@tongji.edu.cn 2 Clean Energy Automotive Engineering Center, Tongji University, 4800 Cao an Road, Shanghai, , P R China Abstract Proper operating range and homogenous distribution are important to proton exchange membrane fuel cell (PEMFC). For the elevated PEMFC water is not suitable to be used as coolant due to high operating. Instead, air can be chosen as coolant because of the relative large difference between fuel cell itself and the ambient air. In this paper, four types of forced air-cooling modes for elevated PEMFC were discussed, and influences of other factors such as cooling air inlet velocity and thermal conductivity of bipolar plate material and fins on heat dissipation and distribution were investigated. Three-dimensional computational fluid dynamics (CFD) method was employed to investigate fluid flow and heat transfer in the elevated PEMFC on a 250 cm2 single cell level. The distribution fields on the active area of MEA were obtained and compared. One optimized mode, with suitable cooling air inlet velocity and bipolar plate material, was determined for future experimental study. Keywords:Elevated, PEMFC, Air-cooling, Cooling structure, CFD, Temperature distribution. 1. Introduction As an energy convertor, proton exchange membrane fuel cell (PEMFC) has many advantages, including clean, efficient and high power density, etc., and it is regarded as an ideal power source for vehicles in the future[1,2]. Operating is a critical factor for PEMFC since it greatly influences the performance and durability of the PEMFC. The ordinary operating of a PEMFC is around 60-80oC. Recent research has confirmed that PEMFC operated at an elevated can bring many benefits, e.g. enhanced electrochemical kinetics for both electrode reactions, simplified water management, and increased CO tolerance[3].however, if the operating is too high, it may lead to degradation on membranes, catalysts layers (CLs) and gas diffusion layers (GDLs). Since energy efficiency of fuel cell is approximately 50%, a kw-class PEMFC stack generates also the same amount of heat. Therefore, it is important to remove the excessive heat and keep PEMFC operating in a proper range. Furthermore, a homogeneous distribution in the active area is also very important for PEMFC. It can increase the kinetic rates at reaction sites and reduce the ohmic losses in the electrolyte [4]. At the same time the electrolyte mechanical strength can also be improved [5]. So a proper cooling structure must be designed to maintain EVS25 World Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 1

2 Page the internal in a suitable range and achieve homogeneous distribution. For PEMFC operating at elevated, i.e oC, water is no longer suitable to be chosen as coolant because the operating is nearly reaching water's boiling point. Thermal oil and air were considered to be chosen as cooling mediums for elevated PEMFC. Till now there are only limited literatures published on high PEMFC stack [6-11], most of which chose thermal oil as coolant because of its higher boiling point and chemical stability at elevated. However, the employment of thermal oil is also problematic. The leaking of thermal oil into the electrode will be lethal to the stack, and leaking to the outside environment will lead to pollution. Therefore the coolant loop should be properly separated from the bipolar plates. And then the stack structure became more complicated and the cost increased. Nomenclature p : Relative pressure (Pa) q : Heat flux(w m -2 ) t : Time(s) T : Temperature u r : Velocity vector (m s -1 ) Greek letters α : Thermal diffusivity (m 2 s -1 ) ρ : Density (kg m -3 ) λ :Thermal conductivity ( W m -1 K -1 ) µ : Viscosity (kg m -1 s -1 ) Subscripts s : Solid f : Fluid Due to the enlarged discrepancy between PEMFC and the ambient air, air can be considered as an optional coolant. Instead of separated and complicated coolant circuit, simplified cooling system or structure can be used, e.g., cooling fins can be added by the bipolar border and then no pump is necessary and disadvantages of oil leakage can be avoided. In the present work, three-dimensional computational fluid dynamics (CFD) method was employed to investigate fluid flow and heat transfer in the PEMFC on a single cell level. The distributions in the active area by virtue of four types of air-cooling modes were obtained respectively, and then compared. Figure 2 Structure of the typical PEMFC geometric model 2.2 Assumptions For modeling purposes, following assumptions were made: 2. Mathematical modeling 2.1 Geometric model The general (or typical) geometric model of the PEMFC is presented in Figure 1. As shown in Figure 2, the geometric model consists of GDLs, bipolar plate, fins, H2/Air flow channels and cooling air flow channels. The fins are designed as elongated parts of the traditional bipolar plate. Cooling air flows over the fins and takes away the heat. Figure 1 Typical geometric model of PEMFC The operating condition is steady; All gases are treated as ideal gas; The incompressible air/h 2 are used as working fluid; Because of lower velocity of gases in flow channel, these zones are regarded as laminar zone; The thicknesses of the membrane and catalyst layer are ignored; The efficiency of the PEMFC is 50%; The heat source is located on the top surface of GDL, which doesn t contact with the bipolar plate, and it generates heat homogeneously; The GDL is porous. 2.3 Governing equations The model in this contribution refers to fluid flow and heat transfer in a steady state, and the gravity influence EVS25 World Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 2

3 Page can be ignored. For the fluid flow, the governing equations are described as follows: Continuity equations r = ( ρ u ) 0 Momentum equation r r r ρ uu = µ u p ( ) ( ) In this model, heat transfer between the forced convection in gases flow and solid heat conduction in the bipolar plate, fins and GDLs are coupled. Therefore the equations for heat transfer in solid and fluid regions are constructed by balancing the energy in a control volume. For the solid regions, i.e. bipolar plate, fins and GDLs, the governing equation of heat transfer is represented as: 2 T s (1) (2) = 0 (3) For the fluid regions, the governing equation of heat transfer is described as: T f t + u T = α T 2 f f f The membrane and catalyst layer are considered as a homogeneous surface heat source, and the heat flow density is given by (4) q = λδ T (5) 2.4 Boundary conditions This paper has investigated the influences on the cooling performance with different structures, different cooling air inlet velocities and different thermal conductivities of bipolar plate and fins. In models, it is assumed continuity at all internal boundaries. The velocity and of the feeding and cooling gases are given at the inlets. Within the active area a constant heat flux is given at the top surfaces of the both GDLs. The contact faces between the fluid domain and solid domain are coupled. All other out walls are assumed to be adiabatic. The detailed geometry parameters are listed in Table 1. Table 1 Parameters for numerical analysis Item Value Electrical power of the fuel cell(w) 120 Heat generation of the fuel cell(w) 120 Active Area(cm 2 ) 250 Length of the bipolar plate(mm) 251 Width of the bipolar plate(mm) 163 Thickness of the bipolar plate(mm) 3.5 Thickness of the fins(mm) 1 Heat exchange area per fin(m 2 ) Cooling medium 1.956e-2 Air Heat flux per GDL top surface(w m -2 ) 2400 Anode gas inlet mass flow (kg s -1 ) 2.9e-6 Anode gas inlet 368 Anode gas outlet pressure(pa) 0 Cathode gas inlet mass flow(kg s -1 ) 1.97e-4 Cathode gas inlet 368 Cathode gas outlet pressure(pa) 0 Cooling Air inlet velocity(m s -1 ) 10/20/30 Cooling Air inlet 298 GDL density(kg m -3 ) 440 GDL thermal conductivity, through plane(w m -1 K -1 ) GDL thermal conductivity, in plane(w m - 1 K -1 ) GDL specific heat(j kg -1 K -1 ) 840 Bipolar plate and fins density(kg m -3 ) 1400 Bipolar plate and fins conductivity(w m- 1K-1) Bipolar plate and fins specific heat(j kg - 1 K -1 ) 2.5 Numerical procedure 20/50/ In the simulation, a three-dimensional, single-phase, non-isothermal and heat transfer model is adopted and implemented into a commercial computational fluid dynamics (CFD) package, FLUENTTM, in order to investigate the distribution on the active area of MEA. The solution procedure is based on SIMPLE algorithm with algebraic multigrid (AMG) method [12]. The solution is assumed to converge if the residuals for the velocity are all under and the residual for energy is less than EVS25 World Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 3

4 Page Results and discussion 3.1 Cooling effect with different cooling modes As shown in Figure 3, there are two types of structure, of which fins are constructed at either the longer or the shorter borders of the bipolar plates. According to the different cooling air inlet positions and flow routes, they can be divided into four cooling modes, i.e. mode A, mode B, mode C and mode D. The coolant flow rates and the heat exchange area of those four modes are all the same. A C Figure3 Models of different cooling modes In the simulations, the conductivity of bipolar plate and fins is 100W m-1k-1, and the cooling flow velocity is 20m s-1. The distributions on the active area under different cooling modes are shown in Figure 4. It can be seen that the distribution on the active area of mode C and D is more homogeneous than that of mode A and B. The table 2 shows that both of the average and highest of mode D is the lowest in the four modes. That means, with the same heat exchange area and coolant flow rate, the cooling effect of mode D is better than the others. A C D B D B Table 2 Indexes of distribution with different cooling modes Cooling mode Average Highest Standard deviation of distribution Mode A Mode B Mode C Mode D Cooling effect with different cooling flow velocities The cooling air flow velocity is an important parameter for the cooling system design, which can determine the type and the power of the cooling fan. Table 3 lists the cooling effect at different cooling air inlet velocities under mode C. The results illustrate that the declines with the cooling flow velocity increasing. Table 3 Indexes of distribution with different cooling flow velocities Flow velocity Average Highest (m s -1 ) Cooling effect with different thermal conductivities Traditionally, the most widely used materials for bipolar plate are the graphite/polymer composites, which are ideal in terms of corrosion resistance and conductivity [13, 14]. Because of high mechanical strength and easily forming, metal such as aluminum, stainless steel becomes a new option as the bipolar plate materials [15]. Due to different properties of these materials, the thermal conductivity varies widely. The distribution s indexes with different thermal conductivities under mode C are presented in table 4. It shows that the cooling effect improves with the increasing of the bipolar and fins thermal conductivity, and the decreases obviously and the distribution becomes more homogeneous. Figure 4 Temperature distributions on the active area under mode A, B, C, D, respectively EVS25 World Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 4

5 Page Table 4 Indexes of distribution with different thermal conductivities Thermal conductivity (W m -1 k -1 ) Average Highest Standard deviation of distribution Conclusions Three-dimensional fluid flow and heat transfer analysis of an elevated PEMFC cooling structure was carried out using CFD software FLUENTTM. The cooling performance of different cooling modes, different cooling air inlet velocities and different thermal conductivities were obtained and compared. For air cooling design, mode D is the best choice, by which excessive heat can be removed more effectively than others. Furthermore, increasing the cooling air inlet flow rate and the thermal conductivity of the bipolar plate and fins can improve the cooling performance and achieve more homogeneous distribution. The experimental results of these fluid flow and heat transfer in the elevated PEMFC models are not readily available to compare the simulation results. In the next step, experimental works will be done with purpose of validation test of the modeling and simulation results. As an important measure, thermocouples will be employed to verify the distribution inside the cell. Acknowledgment The authors would like to thank funding from China Ministry of Science and Technology (grant number: 2008AA050403). Many thanks to Bing LI, Zhiyong LI and Jing FU for providing good advise supports and discussions. References [1] J. Larminie and A. Dicks, Fuel cell systems Explained, Second Edition, West Sussex: Wiley, [2] G.Karimi, J.J.Baschuk and X.Li. Performance analysis and optimization of PEM fuel cell stacks using flow network approach. Journal of Power Sources, Vol. 147, No. 1-2, Sept. 2005, pp: [3] J.L. Zhang, Z. Xie, J.J. Zhang, Y.H. Tang, C.J. Song, T. Navessin, Z.Q. Shi, D.T. Song, H.J. Wang, D.P. Wilkinson, Z.S. Liu, S. Holdcroft, High PEM fuel cells, Journal of Power Sources, Vol. 160, No.2, Oct.2006, pp: [4] F.C. Chen, Z. Gao, R.O. Loutfy and M. Hecht, Analysis of Optimal Heat Transfer in a PEM Fuel Cell Cooling Plate, Journal of Fuel cells, Vol. 3, No.4, Jan.2004, pp: [5]Y.L.Tang, A.M.Karlsson, M.H.Santare, M.Gilbert, S.Cleghorn and W.B.Johnson, An experimental investigation of humidity and effects on the mechanical properties of perfluorosulfonic acid membrane, Materials Science and Engineering, Vol. 425, No.1-2, Jun.2006, pp: [6] L,J.Bonville, H.R.Kunz, Y.song, A.Mientek, M. Williams, A.Ching and J.M. Fenton, Development and demonstration of a higher PEM fuel cell stack, Journal of Power Source, Vol. 144, No.1, Jun.2005, pp: [7] J.O.Jensen, S.Y.Andersen, T.D.Rycke, M.Nilsson, and T.Christensen, High Temperature PEM Fuel Cell Final report, PSO project:4760, 2006 [8]J.Burfeind, G.Bandlamudi, F.Fliusch, C.Siegel and A. Heinzel, Kompakte 140 Wel HT-Brennstoffzelle zum Betrieb mit Methanolreformat, 4. Deutscher Wasserstoff Congress, Feb.2008 [9]J.Scholta, M.Messerschmidt, L.Jorissen and C.Hartnig, Externally cooled with high polymer electrolyte membrane fuel cell stack, Journal of Power Source, Vol. 190, No.1, May. 2009, pp:83-85 [10]P.Pfeifer, C.Wall, O.Jensen, H.Hahn and M.Fichtner, Thermal coupling of a high PEM fuel cell with a complex hydride tank, International Journal of Hydrogen Energy, Vol. 34, No.8, May.2009, pp: [11]J.Scholta, W.B.Zhang, L.Joerissen and W.Lehnert, Conceptual Design for an Externally Cooled HT-PEMFC Stack, The Electrochemical Society,Vol. 12, No.1, 2008, pp: [12]Fluent Inc. FLUENT 6.2 Documentation [13]P.L. Hentall, J.B. Lakeman, G.O. Mepsted, P.L. Adcock and J.M. Moore, New materials for polymer electrolyte membrande fuel cell current collectors, Journal of Power Sources, Vol. 80, No. 1-2, Jul.1999, pp: [14] J. Scholta, B. Rohland, V. Trapp and U. Focken, Journal of Power Sources, Investigations on novel low-cost graphite composite bipolar plates, Vol. 84, No.2, Dec.1999, pp: [15] A.Pozio, F.Zaza, A. Masci and R.F.Silva, Bipolar plate materials for PEMFCs: A conductivity and stability study, Journal of Power Source, Vol. 179, No.2, May 2008, pp : Author Master Candidate. Xiaoliang Zheng School of Automotive Studies, Tongji University, 4800 Cao an Road, Shanghai, P R China, Tel: Fax: morgen_zh@yahoo.com.cn Ph.D. Daijun Yang School of Automotive Studies, Tongji University, 4800 Cao an Road, Shanghai, P R China, Tel: Fax: yangdaijun@126.com EVS25 World Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 5

6 Page Professor. Jianxin Ma School of Automotive Studies, Tongji University, 4800 Cao an Road, Shanghai, P R China, Tel: Fax: jxma@tongji.edu.cn EVS25 World Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 6